I don't remember the numbers (90% is probably a bit exaggerated) but our savings of going from Atlas to MongoDB Community on EC2 several years ago were big.

In addition to direct costs, Atlas had also expensive limitations. For example we often spin up clone databases from a snapshot which have lower performance and no durability requirements, so a smaller non-replicated server suffices, but Atlas required those to be sized like the replicated high performance production cluster.

Was it? Assuming an M40 cluster consists of 3 m6g.xlarge machines, that's $0.46/hr on-demand compared to Atlas's $1.04/hr for the compute. Savings plans or reserved instances reduce that cost further.

Highly doubt that. MongoDB has 5000 well paid employees and is not a big loss making enterprise. If most of the cost was pass through to AWS, they’d not be able to do that. Their quarterly revenue is $500M+ but also spend $200M in sales and marketing and $180M in R&D. (All based on their filings)